In radiation therapy, the use of proton beams provides the possibility of better dose conformity to the treatment target and normal tissue sparing compared to commonly used photon beams because of the lower entrance dose, sharper penumbra and rapid fall off beyond the treatment depth, which result from the Bragg peak in the dose distribution. Despite the dosimetric superiority and some encouraging clinical results for well-localized radio-resistant lesions, the utilization of proton therapy has lagged behind therapies using photons and electrons because the facilities of proton therapy employing cyclotron and synchrotron technology are expensive and complex. As a result, proton therapy has not been a widespread modality in radiation therapy. This situation can be improved if a compact and economical laser-proton therapy unit is available. Laser-proton systems for radiation therapy are currently being developed at the Fox Chase Cancer Center, Philadelphia, Pa. by the present inventors. A typical laser-proton system design includes three types of components: (1) a compact laser-proton source to produce high-energy protons, (2) a compact particle selection and beam collimating device for accurate beam delivery, and (3) a treatment optimization algorithm to achieve conformal dose distributions using laser-accelerated proton beams.
Laser acceleration of particles was first proposed in 1979 for electrons. Rapid progress has been made in laser-electron acceleration in the 1990s since the advent of chirped pulse amplification (CPA) and high fluence solid-state laser materials such as Ti:sapphire. Recently, there have been a number of experimental investigations, which observed protons with energies of several tens of MeV. A recent experiment conducted at Lawrence Livermore National Laboratory reported particles with a maximum energy of 58 MeV. The mechanism for laser-proton acceleration is under study. It has been long linked to the longitudinal electric field created as a result of laser-matter interaction. Recent experimental investigations as well as the results of computer simulations (specifically particle in cell) of the laser-plasma interaction for proton acceleration have shown that laser-accelerated proton beams have broad energy and angular distributions and cannot be directly used in therapy.
A spectrometer-like particle selection and beam modulation system is described by several of the present inventors in which a magnetic field distributed as a step function was used to spread protons in space according to their energies and emitting angles. A particle selection and beam modulation system has been disclosed in International Patent Application No. PCT/US2004/017081, filed Jun. 2, 2004, entitled “High Energy Polyenergetic Ion Selection Systems, Ion Beam Therapy Systems, and Ion Beam Treatment Centers”, the entirety of which is incorporated by reference herein. Subsequently, the proton beams are retrieved with resultant energies, which can be used to generate modulated energy distributions that will deliver the spread-out Bragg Peaks (SOBP). Therefore, the earlier proposed particle selection system constitutes a selection device, which is based on the ideal step field configuration. As a step field distribution is difficult to achieve, further improvements to-particle selection systems that incorporate non-ideal step field configurations are presently needed. Also because non-step field configurations arise from the use of typical electromagnet systems, improvements in the electromagnet systems are currently sought for the efficient and compact separation of laser-accelerated polyenergetic positive ions.